In this study, the T-cell cloning assay will be used to isolate hprt mutants in preterm infants. Mutant frequencies will be calculated in preterm infants of varying gestational ages (24-36 weeks). Through DNA analysis specific types of hprt mutations will be found. A questionnaire of social history and review of the medical history of the mother will be taken as well, which will be related to changes in mutant frequency in preterm infants. This will be compared to mutant frequencies known for full term infants and children. Data are being analyzed using the CDMAS resource.